This invention relates to pressure exchangers for transfer of energy from one liquid flow to another. More specifically, this invention relates to pressure exchangers having an integral pump for the transfer of energy from one liquid stream to another.
The present invention provides a device that can be appropriately described as an engine for exchanging pressure energy between relatively high and relatively low pressure fluid systems, which the term fluid being defined here as including gases, liquids and pumpable mixtures of liquids and solids. The engine for pressure energy exchange of the present invention is a highly efficient device with well over 90% of the energy of pressurization in a pressurized fluid system being transferred to a fluid system at a lower pressure. The device employed for achieving this highly efficient transfer has a long and trouble free operating life which is not interrupted by the plugging and fouling of valves, or the binding or freezing of sliding pistons or the like.
In accordance with the prior art, a typical application of such a pressure exchange apparatus required the use of externally fitted boost pumps to assist the flow of the fluid through the process. This approach would require the use of two separate motors, additional plumbing fittings and often separate power supplies. The present invention provides a device that provides both the pressure exchange function and the boost pump function in a single, efficient package. This approach reduces the need for separate motors, reduces the plumbing requirements and power supply requirements. In this fashion, a system employing the present invention will be less expensive to set up, more reliable and less costly to maintain.
In some industrial processes, elevated pressures are required only in certain parts of the operation to achieve the desired results, following which the pressurized fluid is depressurized. In other processes, some fluids used in the process are available at high pressures and others at low pressures, and it is desirable to exchange pressure energy between these two fluids. As a result, in some applications, great improvement in economy can be realized if pressure exchange can be efficiently transferred between the two fluids.
By way of example, a pressure exchange engine finds application in the production of potable water using the reverse osmosis membrane process. In this process, a feed saline solution is pumped into a membrane array at high pressure. The input saline solution is then divided by the membrane array into super saline solution (brine) at high pressure and potable water at low pressure. While the high pressure brine is no longer useful in this process as a fluid, the pressure energy that it contains has high value. A pressure exchange engine is employed to recover the pressure energy in the brine and transfer it to feed saline solution. After transfer of the pressure energy in the brine flow, the brine is expelled at low pressure to drain.
Accordingly, pressure exchangers of varying design are well known in the art. U.S. Pat. No. 3,431,747 to Hashemi et al. teaches a pressure exchanger for transfer of pressure energy from a liquid flow of one liquid system to a liquid flow of another liquid system. This pressure exchanger comprises a housing with an inlet and outlet duct for each liquid flow, and a cylindrical rotor arranged in the housing and adapted to rotate about its longitudinal axis. The cylindrical rotor is provided with a number of passages or bores extending parallel to the longitudinal axis and having an opening at each end. In accordance with the prior art, a boost pump is employed to reintroduce pressure exchanged fluid into the filtration system. As mentioned previously, this boost pump is a stand alone device employing a separate motor and additional plumbing.
Describing this filtration system in more detail, refer to FIG. 3 which shows a filtration system in accordance with the prior art. A salt water filtration system 300 is shown that uses a reverse osmosis process for the production of potable water which comprises a pressure exchange device 10a in accordance with the prior art.
An unfiltered salt water reservoir 201 provides a supply of unfiltered salt water which is pumped to a high pressure pump 204 by reservoir pump 202. Typically the reservoir pump 202 supplies unfiltered salt water to both the high pressure pump 202 and the pressure exchange device 10a at approximately 30 psi pressure at approximately equal flow rates. The high pressure pump 204 boosts the pressure to approximately 1000 psi and supplies the unfiltered salt water to a filter element 208. In this particular application, and not by way of limitation, the filter element 208 comprises a reverse osmosis type filter device which removes the impurities from the water and provides a fresh water supply 210. A pressure drop occurs in the filter element 208 such that a supply of waste water 209 exits the filter element 208 at approximately 980 psi. Rather than dump this waste water 209 at this elevated pressure, the waste water 209 is supplied to a high pressure inlet 104 of the pressure exchange device 10a. This high pressure waste water is thus used to pressurize additional unfiltered salt water for use in the filtration process. Reuse of the pressure energy contained in the high pressure waste water 209 thus provides for a highly efficient filtration system 200.
As mentioned previously, the reservoir pump 202 supplies unfiltered salt water to a low pressure inlet 100 of the pressure exchange device 10a. The pressure exchange device 10a is configured to raise the pressure of the unfiltered salt water supplied to it by the reservoir pump 202 to a pressure equal to the pressure of the waste water 209 supplied to the high pressure inlet 104.
A high pressure outlet 106 located on the pressure exchange device 10a is in fluid communication with a separate boost pump 214. The pressure energy of the waste water 209 from the high pressure outlet 106 is supplied to the separate boost pump 214 for example at approximately 960 psi and the boost pump 214 raises the pressure to the high pressure pump discharge pressure and supplies the unfiltered salt water to the filter element 208 for filtration. Thus, a closed loop system is provided that maximizes the use of the waste water and reuses the high pressure of the waste water to increase system efficiency.
However, the use of a separate boost pump in accordance with the prior art has proven problematic and costly. A separate pump reduces overall system reliability and also increases operating and fabrication costs.
There therefore is a need for a pressure exchanger which provides for an integral boost pump feature.